A laser printer toner cartridge recycling operation can liberate hundreds, or even thousands of pounds of spent toner per hour. Unfortunately, in most such operations, a portion of the toner becomes airborne and can be a health hazard if it is inhaled by persons, or it can be an explosion or fire hazard. For safety, it is vital that operators control these hazards. This is traditionally accomplished by high volume dry filtering of toner laden air. Modern dry air filtering systems, e.g. bag houses, have good health and safety records, but only when they are properly designed, installed and maintained. These systems consist of hoods, ducts, fans, filters, bags, etc. They are complex, expensive to build and install and they require constant vigilance and maintenance to meet required levels of safety and performance.
Wet scrubbers are an alternative means for removing airborne dust. They generally have lower capital and maintenance costs comparable bag houses. Wet scrubbers are successfully used in coal fired electrical power generation facilities to remove fly ash from exhaust gasses. They also are successfully used in many other applications however; conventional wet scrubbers do not perform well in for removing airborne laser printer toner. Still, when the reasons for their poor performance in a toner application can be identified and corrected, they offer greater margins of health and safety and lower overall cost than comparable dry filtering systems. After scrubbing, toner is removed from the operating fluid and collected as damp cake or mud. Wet toner can be safely handled and used because of its decreased ignitability and the reduced probability for an accidental fire or explosion. Finally, wet scrubbing can be easily setup as a continuous process. That is a significant advantage when wet scrubbers are part of a continuous production operation. These attributes make wet scrubbers inherently safer, as well as, more economical compared to bag houses.
Commercial wet scrubbers, suitable for collecting airborne toner, are not currently available. Conventional scrubbers collect dust in the operating fluid as sludge. These scrubbers handle dust whose density is higher than that of the operating fluid very well. However, they do not handle dust that floats, e.g. density less than that of the operating fluid, nearly so well. Low cost, non-flammable operating fluids are usually water based with specific gravities near 1.0. Toner consists mostly of low melting point plastic that has a specific gravity less than most water based operating fluids, i.e. much of the toner floats. However, toner also contains a significant amount of other higher density materials such as pigments, desiccants, ferromagnetic material, performance enhancers, etc. Thus, a successful scrubber must handle materials with densities above and below that of its operating fluid.
The engineering problem is to successfully remove high volumes of floating and sinking materials while preserving the filtering performance and the health and safety benefits of wet scrubbing. Extensive analysis, testing, design and development were required to build a satisfactory wet scrubber to meet these requirements.